Pattern matching an array of n items

I'm testing a model in rails, and I'd like to assert that one of the associations has an n items. I've found that the assert_pattern matcher is a pretty good way to do it:

assert_pattern do  author.books.to_a =>    [Book,    Book,    Book,    Book,    Book]end

However, I've also found that having a big list of items like that can be difficult to read. I've tried expanding the array the way I usually would in ruby:

assert_pattern do  author.books.to_a => [Book]*5end

However, within a pattern matcher, that's a syntax error:

test/models/book_test.rb:39: syntax error, unexpected '*', expecting `end' or dummy end (SyntaxError)    author.books.to_a => [Book]*5                               ^

Is there a concise way to make a pattern that's a function of length?